1. Field of the Invention
The present invention relates generally to hysteretic buck converter control schemes, and more specifically, to a buck converter control circuit in which an indication of output current is used to adjust turn-on timing dynamically.
2. Background of the Invention
At low output current levels, pulse width modulator (PWM) controllers and other types of switching power regulators that deliver high current levels under high load conditions are inefficient. Since the pulse width becomes very narrow for low output current levels, the power used to operate the switching circuits and control/sensing circuits, which does not typically change with load current demand, predominates converter power consumption, making the converter very inefficient during low demand conditions. Alternative controller modes are frequently implemented to operate the converter in a standby low power mode, in which the full converter dynamic performance is not available, but a minimum output voltage is maintained to provide required power supply voltage(s) when the load current demand is low. Pulse-frequency modulator (PFM) circuits are frequently used in low power operating modes, as the pulse frequency can be arbitrarily reduced based upon load demand. Hysteretic control circuits, in which the output voltage is maintained between two predetermined set points, have been applied to provide such low-power operating modes. Hysteretic converters have a wide dynamic range and potentially low power consumption, due to their activation only when the output voltage falls below an acceptable limit.
Also, in low power applications in which either the complexity or the power required for PWM operation is undesirable, hysteretic controllers are sometimes used to provide the power supply control algorithm for all levels of output current, since the control circuit itself can be placed entirely in standby mode, with the low-limit voltage sensing circuit being the only circuit required to operate. The output of the low-limit voltage sensing circuit can then activate the remainder of the converter when the output voltage must be raised. Further, in any application in which the transient response of a PWM converter is not sufficiently fast for responding to load transients, hysteretic converters are also used to provide a fast response to changing load conditions.
In typical hysteretic converters, a constant-width pulse is provided when the output voltage falls below a low-limit threshold, injecting a charge into the output capacitor that raises the output voltage by a predetermined amount. However, if the output current or input voltage conditions are changing, such a converter can produce an undesirable level of ripple, as the constant-width pulse is not responsive to different levels of load current or input voltage. In other types of hysteretic converters, the input voltage and other power supply conditions are monitored and the width of the output pulse is controlled so that the level of ripple is controlled to a greater degree than the constant-width controllers can provide.
However, each of the above hysteretic controllers, load transients or input voltage droop can cause undershoot of the low-limit threshold due to time required for the converter to respond. In the constant-width converter, several pulses may be required for the output voltage to recover and in the width-controlled hysteretic converter, an initial undershoot is present, which is corrected by the pulse that has been triggered.
Therefore, it would be desirable to provide a power supply circuit and control method that reduce ripple in a hysteretic converter by controlling undershoot.